Mmm! It does sound as though something has occurred between your husband and Dil but it could be anything.
Did your son go out with them on the evening you were ill? You say that Dil and Dh went out for the night and don't mention your son.
Maybe they had an argument and that has caused the tension between them and your husband with you.
I would ask your Dil, face to face, had there been a falling out between them on holiday, as you'd noticed some tension between them and watch her face for a reaction.
The other scenario, is that your Dh had made a fool of himself with her, in some way.
Unfortunately, I know two women, whose "silly old fools of a" Fil, has made a pass at them, both times after quite a lot of drinking.